Continuous inkjet printing involves the formation of electrically charged drops from a jet of ink, and the subsequent deflection of the charged drops by an electric field to produce an image on a print medium. In a typical embodiment of a single jet printer of this type, electrically conducting ink is forced through a nozzle by applying pressure to the ink. The velocity of the jet requires control; often achieved by control of the constituency of the ink in conjunction with controlling the pressure. Pressure control is usually achieved by varying the speed of the pump producing the flow with feedback from a pressure transducer. It has also been achieved using feedback from a velocity measurement device.
Within a typical continuous inkjet printer, ink is re-circulated constantly from, and back to, an ink reservoir. A small proportion of the circulating ink is lead off to the print head feed line and, of the ink passing through the feed line to the print head, the un-printed ink drops are collected in the print head gutter and returned to the ink reservoir as well.
Keeping the constituency of the ink at a constant composition is normally achieved by controlling its viscosity, and adding one or more solvents to replace the solvents that have evaporated from running the continuous jet.
Many attempts have been made to reduce the amount of solvent consumed. For example European Patent Application No. 0 123 523 (Willet) and European Patent Application No. 0 560 332 (Hitachi) both disclose re-circulating the air returned from the gutter to the reservoir so that, after a short period of time, the air in the print head is saturated and the loss of solvent is minimised. A similar system is disclosed in U.S. Pat. No. 4,283,730, which describes the recirculation of solvent laden air to the print head. International Patent Application WO2008117013 (Linx) discloses feeding a portion of air back to the head because feeding solvent-laden air back into the print head can cause problems at low temperature e.g. where solvent-laden air condenses onto the print head electrodes causing failure. The outlined solution is to allow part of the air to be vented to atmosphere rather than back to the print head and to place the recirculation pipe close to the gutter.
Another problem with re-circulating gutters is that, if the return pipe becomes blocked, there is a tendency for the reservoir to become pressurised and to burst as air is drawn in by the gutter pump and can't escape.
European Patent 0 076 914 purports to describe a continuous inkjet printer in which the evaporation rate of the ink is low. In the embodiment described a heat exchanger is provided in the ink feed line, just upstream of the drop generator, and this is said to cool the ink and, thereby, reduce evaporation of the ink. No description is provided of any system to add solvent to the ink but, in any event, the apparatus described will have negligible effect on the temperature of the ink, and any solvent consumption, because the ink flow through the print head is extremely low. Typically, of the ink circulating within the printer, approximately 0.5% of the flow is directed through the print head. Still further, current continuous inkjet printers include a heating facility to heat the ink and obviously such a facility makes the arrangement shown in EP 0 076 914 pointless.
A further method used to reduce solvent consumption is to use a Peltier cooler in the vent line from the ink reservoir. The Peltier cooler condenses solvent and returns it either to the ink reservoir or to the make-up reservoir. A Peltier cooler is used for example on the applicant's A-Series printer. Although this method is effective, Peltier coolers are inefficient and expensive.
It is an object of the invention to provide a continuous inkjet printer which will go at least some way in addressing the aforementioned problems; or which will at least offer a novel and useful alternative.